Visit any flying site and you’ll notice that, whether i.c. or electric powered, the majority of models being flown are single-engine. That’s not to say that twins aren’t seen, but for many the journey into multi-engine installations is a trip into the unknown and, therefore, often avoided like the plague!
One reason for this is probably the cost, as models with multiple power units are, by nature, more expensive to build and operate. However, back in my i.c. days the main deterrent to flying a twin was the ever-present risk of one engine cutting out and having the nightmare of dealing with the resulting asymmetric thrust, which often led to the suffering aircraft’s demise.
Without doubt, modern i.c. engines are much more reliable than they used to be, although the risk of a dead engine still exists, with fuel issues and failing glow plugs probably accounting for the vast majority of cases. With an electric model, the chance of just one motor stopping is substantially lower and would normally require failure of a major powertrain component such as an ESC or motor, which is extremely rare if these are being operated within their recommended limits. So, from a reliability perspective, anyone considering a multi-engine model as a future project would be much safer in opting for electric power.
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However, there are a range of options and pitfalls to consider when setting up an electric multi, to the point that this added complexity is another good excuse to ‘stay single’, as it were! Anyway, hopefully I’ll be able to simplify things here and demonstrate that, if approached correctly, there really isn’t much to it.
Fig.1
THE BIG BRUSH-OFF!
It seems rather odd to describe electric flight setups with brushed motors as being out of date, however the performance advantage and increasingly low cost of brushless technology over recent years has meant that brushed motors have been all-but forgotten. This is a little sad as brushed setups did have certain advantages, not least when it came to multi-engine models. The reason for this is simple: one ESC could drive all of the motors! This is because a brushed motor simply runs faster or slower depending on how many volts are passed through it, and the ESC isn’t bothered if it sends this voltage to one or many motors, providing the total current consumed is within the capacity of both ESC and battery. Such simplicity is why some of today’s four-engine models still use brushed setups with geared motors (a gearbox increases a motor’s efficiency considerably).
Of course, there are still wiring options to be considered for such a solution; the system can be wired either in parallel (Fig.1) or series (Fig.2), depending on how many cells you wish to use. Series is better for larger numbers of cells, with parallel being preferable for lower cell counts.
Back in the day, most of these setups used the best NiCd and NiMH cells that were available at the time, but there’s no reason why you can’t use Li-Pos here, providing the brushed ESC has programmable voltage cut-off or you incorporate a separate Li-Po saver in the circuit to make sure they don’t fall below their critical voltage (usually deemed to be 2.5V per cell as an absolute minimum).
Fig. 2
BRUSHLESS MULTIS
With brushless multis, confusion seems to originate from the core requirement. Each power unit (whether a motor or EDF unit) has to have its own, dedicated ESC. This is necessary because modern sensorless speed controllers rely on a tiny feedback signal from the motor windings in order to recognise the motor’s speed and direction and therefore be able to control it. When using more than one motor with a single ESC there are two (or more) feedback signals being sent to the ESC, which confuses it and can cause all manner of problems.
Kontronik managed to get past this complication with its twin 400 power-set, but to my knowledge no other system has been marketed with the same facility so we’re stuck with one ESC per motor for the time being.
SPAGHETTI JUNCTION
This ‘one ESC per motor’ arrangement causes considerably more wiring and has to be carefully considered, as there are several options. I’ll start with the easiest and most common system used, and then expand into more complicated ways of doing it. There’s a lot to take in, but at some stage in your enjoyment of electric flight you may embark on a multi-engine model project, and this will hopefully provide useful reference when you need it.
Fortunately, accessory manufacturers have recognised our need to vary our setups and there’s now a huge range of leads available for almost every permutation. That said, if you’re handy with a soldering iron you can save yourself a good bit of money by making up your own leads from a pack of connectors and some decent wire.
Fig. 3
1 BATTERY, 2 MOTORS
This is the simplest, and in many ways the safest, way to operate a model with twin power units and is ideal for models where the motors are wing-mounted. Basically, the flight battery is connected to the two ESCs using what is effectively a high current Y-lead, with each ESC being wired to its motor as normal. Needless to say, the ESCs, motors and props need to be of identical stock (see Fig.3).
This setup ensures that apart from failure of an ESC or motor, both motors should run at theoretically identical rpm from the outset, and will reduce equally as the flight battery voltage decays. In most cases even the low voltage cut-off will occur at the same time (or at worst within the space of a couple of seconds), elim
inating the chance of asymmetric thrust problems. Mind you, in practice most would land the model at the first sign of the performance dropping off anyway, whilst keeping a little in hand just in case.
Each ESC needs to be of sufficient current handling capacity for the motor and prop being used, but the flight battery needs to be capable of handling the sum of each side, i.e. twice the current of a single motor. For this reason you’ll find setups like this using a large capacity (e.g. 4000mAh) Li-Po or larger when running a pair of motors drawing (for example) 20A each. The total current here is 40A, which equates to a 10C loading on the Li-Po – good for battery longevity. Incidentally, the ESCs will also need a normal Y-lead plugged into the motor channel of your Rx.
Fig. 4
2 BATTERIES, 2 MOTORS
Of course, many modellers use popular sport-size 3s Li-Pos of around 2200mAh and would prefer to use these rather than purchase a single, large capacity pack for use in the setup described above. There are two different ways of using a pair of smaller packs to do effectively the same job, and the method chosen has much to do with the type of model the setup is being used in.
The first – best for twins with outboard power units – is simply to join a pair of the aforementioned 2200mAh Li-Pos in parallel, using a suitable lead, effectively making it into a single 4400mAh pack, and using this in the manner shown in Fig.4.
The other way of using two motors with two batteries is to incorporate them as two individual powersets, the only link between them being the ESCs via a common Y-lead (Fig.5).
This is more often used in models where the power units are very close together, such as a twin EDF jet where the nozzles exit the fuselage side-by-side, i.e. Tornado, F-15 etc. In this situation, clearly, asymmetric thrust caused by one Li-Po going flat before the other will have a less dramatic effect. In practice, with identical motors, ESCs and Li-Pos from the same batch with the same cycle life, it’s unlikely that this would happen. That said, Li-Pos can be an unpredictable breed at times and the parallel route would probably be safer all round.
Fig. 5
2 BATTERIES, 4 MOTORS
When it comes to models using four motors, unless these are relatively small it’s unlikely that a single Li-Po, even of the largest capacity, will be suitable. I’ve seen an electric Lancaster with four brushless 400-size motors running off a single 5400mAh 3s Li-Po with great success, although each motor was drawing less than 15A and so allowed the single pack to cope.
In most cases, though, at least two flight batteries will be required. However, there are ways of wiring the setup with asymmetric safety in mind, even if the wiring may seem a little complicated (Fig.6).
Using the Lancaster as a classic example, the last way you’d want to wire such a model would be with one battery running both port motors and the other running both starboard motors, for what I hope are obvious reasons! The way to go is to use one battery to power both inboard motors, with the other powering both outboard motors. By doing this there’s no necessity for the motors – or even the flight batteries for that matter – to be the same specification as they can be regarded as completely different systems, as long as each ESC and motor pair is identical. However, always use the more powerful of the two setups on the inboard with the less powerful on the outboard, just like on an i.c. model that may have a pair of .40s for inboard and a pair of .25s outboard.
Having two flight batteries in a setup like this can also be handy when it comes to positioning for C of G, as they’ll normally go where one large battery may not – although keeping wire lengths as short as possible is always good practice.
One thing that can get a little messy when using four ESCs is that they all need to use one common throttle channel at the Rx, and this can require three Y-leads to get four plugs down to one. I’m sure that someone must produce a 4-into-1 lead for this purpose, and if so it would be well worth getting hold of, if just to reduce the amount of wiring in the model. This multi-power unit concept had to be dealt with by Graham Dorschell when finalising the set-up of his EDF Vulcan featured last issue. I’d hoped to bring you the full spec’ of the Vulcan this month, but with Graham being in Thailand at present that hasn’t happened. No matter, it will follow as soon as he’s back on home soil.
Fig. 6
BEC CONSIDERATIONS
One thing that does cause trepidation when using more than one ESC is what to do about supplying power to the Rx and servos. This is fairly simple in a single engine model, since all you need to do is work from the ESC’s specification. If the amount of servos being used in the model is within the unit’s limit for the input voltage (i.e. the number of cells being used) then it’s happy days. If it’s outside that limit then a separate, stand-alone BEC unit will be required, and the red lead to the Rx snipped and isolated.
However, when using multiple ESCs, all sorts of questions arise. For example, will one ESC’s BEC circuit burn the other out? Can they work in harmony? Is the BEC current handling capacity cumulative? Should I snip and isolate every red wire and use a stand-alone BEC unit anyway? It really is a grey area!
If you have any doubt, always use a separate Rx power supply. This doesn’t necessarily have to be an electronic unit, it can just as easily be a NiMH pack like the ones i.c. flyers use. In many ways this is a better solution as it’s completely independent of the flight batteries and therefore won’t use any of their capacity. As a result, flight times will be longer for servos can use more capacity than you might imagine, especially where a model has several digitals on board. When using a separate battery you do indeed have to snip and isolate each red wire from the ESC, as the four BECs will actually attempt to charge the battery and it can become quite warm, so be careful here.
My own experience is that when using two, or even four, identical ESCs, each with a BEC, they all appear to work in unison with each other and their capacity is indeed cumulative, so using four 2A BECs will give a total of 8A. Some say this is wrong, and that the whole set-up is at risk of melting(!). And so the BEC debate smoulders on (no pun intended), and you simply have to go with what you believe. But again, isolating the BECs and using a separate Rx power supply has to be the safest all-round way of doing things.
